Virulent M. tuberculosis is a facultative intracellular parasite which multiplies freely within the alveolar macrophages in the normal lung. On the other hand the avirulent strain H37Ra is progressively eliminated from the normal mouse lung with time. A number of recombinant strains of M. smegmatis and H37Ra bearing DNA fragments from virulent H37Rv are being tested in intravenously infected mice and the results indicate that some recombinant strains are capable of persisting within the lungs of intravenously infected mice better than controls bearing only the phasmid DNA. These in vivo selected recombinant H37Ra strains will be tested for relative growth/survival in mouse peritoneal, splenic and alveolar macrophage cultures as well as tissue culture compared to virulent H37Rv and parental H37Ra controls. Macrophages will be infected in vivo or in vitro and the growth compared for macrophages harvested from C57Bl/6 (Bcgs) and A/J(Bcg) mice. The resulting growth curves will be compared with those observed using the J774 cell line. Those recombinants which show maximum intracellular growth will be examined for the presence of the 25 kb genomic fragment containing the ivg gene shown earlier to confer a growth advantage to in vivo selected recombinant strains of M. tuberculosis.